There are three common spoke nipple sizes used on bikes and Park sells a set of three reasonably priced color-coded wrenches that should cover anything you are likely to see. They are:
Black handled SW-0 (3.23 mm) that fits DT, Wheelsmith and other high quality spokes.
Green handled SW-1 (3.30 mm) that fits Union and a few other European spokes.
Red handled SW-2 (3.45 mm) that fits older Japanese spokes.
Another possibility is Park's SW-10 adjustable spoke wrench that covers all three sizes in one tool.